1. Field of the Invention
The invention relates to means for turning power on and off to a pulsed transmitting module in an array of similar modules used to drive the elements of a phased array radar anenna, and more particularly to a novel means of turning the module on to a precise digitally controlled power level, and then sustaining the power level throughout the pulse.
2. Prior Art
In a conventional phased array radar system, a low power exciter generates the carrier of the transmitted radar signal. The exciter output is modulated in amplitude and/or phase to generate radar pulses of low power. These low power pulses are then distributed in controlled amplitude and phase to an array of power amplifying modules each arranged to drive an antenna element of the phased array. In order to conserve energy, the power amplifying modules are turned on a moment before the modulated exciter pulse begins and are turned off a moment after the modulated exciter pulse ends. The reason for applying power to the modules only for the duration of the exciter pulse is to minimize heating of the power modules for a given peak power, and thereby maximize the peak power.
In a conventional power supply for a radar transmitter, the power supply is designed to handle the average power consumed by the transmitter during intermittent operation with reliance being placed on energy storage in large capacitors to sustain the voltage on the transmitter when a high peak power is required of the supply during the pulse.
The supply voltage can be better sustained with additional energy storage, but only at the cost of added bulk. The term "capacitor droop" has been applied to the effect on the transmitted pulse of a falling supply voltage at the transmitter or power amplifier. Capacitor droop is present to some degree in most radar transmissions. If a single transmitter or power amplifier feeds all the antenna elements, the droop is simultaneous on all antenna elements and the adverse effect on the beam is small. However, when each antenna element has one or a plurality of power amplifiers, and the power amplifiers have potentially different power supplies, then the problem becomes severe. Here the reactive energy storage properties of the supplies must remain matched or the "droop" will not be simultaneous at all elements in the array and the beam will be distorted and pulse-to-pulse correlation severely reduced.
A further common requirement of a power amplifier for the antenna element in a phased array radar is that it be capable of plural outputs to achieve tapering of the r.f. output depending on the position of the element in the array. Tapering may be achieved by adjustment of the supply voltage at each power amplifier. It is therefore desirable that the output of a power supply be conveniently adjustable or programmable.
The transmitting power modules of current design employ solid state components in the power modules driving the antenna elements. Metal Oxide Semi-conductor Field Effect Transistors (MOSFETs) or High Electron Mobility Transistors (HEMT) are commonly used in this application, and such devices are often fabricated on a Gallium Arsenide substrate. At frequencies in excess of 1 GHZ, a Monolithic Microwave Integrated Circuit (MMIC) format, which combines passive circuit features with the active devices on a Gallium Arsenide substrate is common. These circuits are particularly compact.
In the conventional power conditioner for transmitting power modules, a common element is the switch, which turns the power on and off (hence the term "drain switch" by virtue of its connection to the drain electrode of a field effect transistor), accompanied by local inductors or capacitors to sustain the dc when peak power is required. With the advent of higher frequency operation and MMIC fabrication techniques, the MMIC r.f. circuits in the transmitting modules tend to be dwarfed by their associated power supply components. In such applications, it is particularly desirable that the module power conditioner be of minimum bulk.